Resonant Neutrinoless Double-electron Capture Research Articles

Search for Double Beta Decay of 106Cd with an Enriched 106CdWO4 Crystal Scintillator in Coincidence with CdWO4 Scintillation Counters

Studies on double beta decay processes in 106Cd were performed by using a cadmium tungstate scintillator enriched in 106Cd at 66% (106CdWO4) with two CdWO4 scintillation counters (with natural Cd composition). No effect was observed in the data that accumulated over 26,033 h. New improved half-life limits were set on the different channels and modes of the 106Cd double beta decay at level of limT1/2∼1020−1022 yr. The limit for the two neutrino electron capture with positron emission in 106Cd to the ground state of 106Pd, T1/22νECβ+≥2.1×1021 yr, was set by the analysis of the 106CdWO4 data in coincidence with the energy release 511 keV in both CdWO4 counters. The sensitivity approaches the theoretical predictions for the decay half-life that are in the range T1/2∼1021−1022 yr. The resonant neutrinoless double-electron capture to the 2718 keV excited state of 106Pd is restricted at the level of T1/20ν2K≥2.9×1021 yr.

First search for 2ε and εβ+ processes in 168Yb

The double-electron capture and the electron capture with positron emission in 168Yb have been investigated for the first time at the STELLA facility of the Gran Sasso underground laboratory (Italy) measuring 371 g of highly purified ytterbium oxide placed on the end-cap of a 465 cm3 ultra-low-background high purity Germanium detector (HPGe). No gamma associated to double beta processes in 168Yb have been observed after 2074 h of data taking. This has allowed setting the half-life limits on the level of lim⁡T1/2∼1014–1018 yr at 90% C.L. Particularly, a lower half-life limit on a possible resonant neutrinoless double-electron capture in 168Yb to the (2)− 1403.7 keV excited state of 168Er is set as T1/2≥1.9×1018 yr at 90% C.L. Half-life limits T1/22ν(0ν)≥4.5(4.3)×1016 yr were set on the 2ν(0ν)2β− decay of 176Yb to the 2+ 84.3 keV first excited level of 176Hf.

Development of the ultra-low background HPGe spectrometer OBELIX at Modane underground laboratory

A new ultra low-background spectrometer based on a HPGe detector with a sensitive volume of 600 cm3 was developed to investigate rare nuclear processes, such as resonant neutrino-less double electron capture (0νEC/EC) and double beta decay processes (2ν2β−, 2νβ+EC, 2νEC/EC) to the excited states of daughter nuclei. The spectrometer was installed at the Modane underground laboratory (LSM, France, 4800 m w.e.). Sensitivity of the spectrometer and its background were tested. A new method for the efficiency calibration in measurements of low-active samples was developed. The spectrometer was used for the measurements of low active materials and samples. Results obtained in 395 h investigation of resonant 0νEC/EC decay of 106Cd to the 2718 keV and 2741 keV excited states of 106Pd with ∼23.2 g of enriched 106Cd and 2ν2β− decay of 100Mo sample with a mass of 2588 g to the 0+, 1130 keV and 2+, 539.5 keV excited states of 100Ru are presented.

Search for neutrinoless double-electron capture ofDy156

Background: Multiple large collaborations are currently searching for neutrinoless double-$\ensuremath{\beta}$ decay, with the ultimate goal of differentiating the Majorana-Dirac nature of the neutrino.Purpose: Investigate the feasibility of resonant neutrinoless double-electron capture, an experimental alternative to neutrinoless double-$\ensuremath{\beta}$ decay.Method: Two clover germanium detectors were operated underground in coincidence to search for the de-excitation $\ensuremath{\gamma}$ rays of $^{156}\mathrm{Gd}$ following the neutrinoless double-electron capture of $^{156}\mathrm{Dy}$. 231.95 d of data were collected at the Kimballton underground research facility with a 231.57 mg enriched $^{156}\mathrm{Dy}$ sample.Results: No counts were seen above background and half-life limits are set at $O({10}^{16}\text{--}{10}^{18})$ yr for the various decay modes of $^{156}\mathrm{Dy}$.Conclusion: Low background spectra were efficiently collected in the search for neutrinoless double-electron capture of $^{156}\mathrm{Dy}$, although the low natural abundance and associated lack of large quantities of enriched samples hinders the experimental reach.

Theory of neutrinoless double beta decay—A brief review

Neutrinoless double decay (0νββ-decay) is a unique probe for lepton number conservation and neutrino properties. This is a process with long and interesting history with important implications for particle physics and cosmology, but its observation is still elusive. The search for the 0νββ-decay represents the new frontiers of neutrino physics, allowing to determine the Majorana nature of neutrinos and to fix the neutrino mass scale and possible CP violation effects, which could explain the matter-antimatter asymmetry in the Universe. At present a complete theory is missing and, thus, to motivate and guide the experiments the mechanism mediated by light neutrinos is mostly considered. The subject of interest is an effective mass of Majorana neutrinos, which can be deduced from the measured half-life, once this process is definitely observed. The accuracy of the determination of this quantity is mainly determined by our knowledge of the nuclear matrix elements. There is a request to evaluate them with high precision, accuracy and reliability. Recently, there is an increased interest to the resonant neutrinoless double electron capture, which may also establish the Majorana nature of neutrinos. This possibility is considered as alternative and complementary to searches for the 0νββ-decay.

Double beta decays of 124Xe investigated in the QRPA framework

The two-neutrino (2ν2β) and neutrinoless (0ν2β) double beta decays of 124Xe are investigated. Decays to the ground state, , and various 0+ and 2+ excited states in 124Te are considered. The corresponding nuclear matrix elements have been calculated by using the quasiparticle random-phase approximation combined with the multiple-commutator model. G-matrix-based effective nuclear interactions have been used in realistic single-particle model spaces. All possible channels, β+β+, β+EC, and ECEC, are discussed for both the 2ν2β and 0ν2β decays. The associated half-lives are computed, in particular the one corresponding to the resonant neutrinoless double electron capture transition to the 2790.41 keV nuclear state in 124Te. This work represents the most complete theoretical investigation thus far of the double-beta-decay properties of 124Xe.

Nuclear matrix elements for neutrinoless double-beta decay and double-electron capture

A new generation of neutrinoless double-beta decay (0νββ-decay) experiments with improved sensitivity is currently being designed and under construction. They will probe the inverted hierarchy region of the neutrino mass pattern. There is also a revived interest in the resonant neutrinoless double-electron capture (0νECEC), which also has the potential to probe lepton number conservation and to investigate the neutrino nature and mass scale. The primary concerns are the nuclear matrix elements. Clearly, the accuracy of the determination of the effective Majorana neutrino mass from the measured 0νββ-decay half-life is mainly determined by our knowledge of the nuclear matrix elements. We review recent progress achieved in the calculation of 0νββ and 0νECEC nuclear matrix elements within the quasiparticle random phase approximation. A considered self-consistent approach allows us to derive the pairing, residual interactions and the two-nucleon short-range correlations from the same modern realistic nucleon–nucleon potentials. The effect of nuclear deformation is taken into account. The possibility of evaluating 0νββ-decay matrix elements phenomenologically is discussed.

Theoretical investigation of the double-βprocesses in96Ru

The two-neutrino ($2\ensuremath{\nu}2\ensuremath{\beta}$) and neutrinoless ($0\ensuremath{\nu}2\ensuremath{\beta}$) double-$\ensuremath{\beta}$ decays of ${}^{96}$Ru are investigated for the transitions to the ground state, ${0}_{\mathrm{gs}}^{+}$, and ${0}^{+}$ and ${2}^{+}$ excited states in ${}^{96}$Mo by using the quasiparticle random-phase approximation combined with the multiple-commutator model. $G$-matrix-based nuclear forces are used in realistic single-particle model spaces. All the possible channels, ${\ensuremath{\beta}}^{+}{\ensuremath{\beta}}^{+}$, ${\ensuremath{\beta}}^{+}$EC, and ECEC, are discussed for both the $2\ensuremath{\nu}2\ensuremath{\beta}$ and $0\ensuremath{\nu}2\ensuremath{\beta}$ decays. The associated half-lives are computed, in particular the one corresponding to the resonant neutrinoless double electron capture (R$0\ensuremath{\nu}$ECEC) transition to the $2.712.68\text{-MeV}$ nuclear state in ${}^{96}$Mo. This work represents the most complete theoretical investigation of the double-$\ensuremath{\beta}$-decay properties of ${}^{96}$Ru thus far.

Evaluation of the resonance enhancement effect in neutrinoless double-electron capture in152Gd,164Er, and180W atoms

We study the resonant neutrinoless double-electron capture ($0\ensuremath{\nu}$ECEC) in ${}^{152}\mathrm{Gd}$, ${}^{164}\mathrm{Er}$, and ${}^{180}\mathrm{W}$ atoms, associated with the ground-state to ground-state nuclear transitions. The corresponding matrix elements are calculated within the deformed quasiparticle random-phase approximation using the realistic charge-dependent Bonn (CD-Bonn) nucleon-nucleon interaction. The half-lives are estimated with the use of the most recent precision data on the $Q$ values of these processes. Perspectives of experimental search for the $0\ensuremath{\nu}$ECEC with the isotopes ${}^{152}\mathrm{Gd}$, ${}^{164}\mathrm{Er}$, and ${}^{180}\mathrm{W}$ are discussed.

Final results of an experiment to search for 2β processes in zinc and tungsten with the help of radiopure ZnWO4 crystal scintillators

A search for the double beta decay of zinc and tungsten isotopes has been performed with the help of radiopure ZnWO4 crystal scintillators (0.1–0.7 kg) at the Gran Sasso National Laboratories of the INFN. The total exposure of the low background measurements is 0.529 kg × yr. New improved half-life limits on the double beta decay modes of 64Zn, 70Zn, 180W and 186W have been established at the level of 1018–1021 yr. In particular, limits on double electron capture and electron capture with positron emission in 64Zn have been set: T2ν2K1/2 ⩾ 1.1 × 1019 yr, T0ν2ε1/2 ⩾ 3.2 × 1020 yr, T2νεβ+1/2 ⩾ 9.4 × 1020 yr and T0νεβ+1/2 ⩾ 8.5 × 1020 yr all at 90% CL. Resonant neutrinoless double electron capture in 180W has been restricted on the level of T0ν2ε1/2 ⩾ 1.3 × 1018 yr. A new half-life limit on α transition of 183W to the metastable excited level 1/2− 375 keV of 179Hf has been established: T1/2 ⩾ 6.7 × 1020 yr.

Probing the nuclides102Pd,106Cd, and144Sm for resonant neutrinoless double-electron capture

The $Q$ values for double-electron capture in ${}^{102}$Pd, ${}^{106}$Cd, and ${}^{144}$Sm have been measured by Penning-trap mass spectrometry. The results exclude at present all three nuclides from the list of suitable candidates for a search for resonant neutrinoless double-electron capture.

Neutrinoless double-beta decay and related topics

The fundamental importance of searching for neutrinoless double-beta decay (0νββ-decay) is widely recognized. Observation of the decay would tell us that the total lepton number is not conserved and that, consequently, neutrinos are massive Majorana fermions. A brief history of the double-beta decay is presented. The 0νββ-decay is discussed in context of neutrino oscillation data. The perspectives of the experimental 0νββ-decay searches are analyzed. The importance of reliable determination of the 0νββ-decay nuclear matrix elements is pointed out. The problem of distinguishing of the light-neutrino exchange, heavy-neutrino exchange and the trilinear R-parity breaking supersymmetric (\(\not R_p \) SUSY) mechanisms of the 0νββ-decay is addressed. Further, the process of resonant neutrinoless double-electron capture (0νɛɛ) is revisited. Arguments are presented that an experimental search for the 0νɛɛ might be feasible.

Neutrinoless double beta decays of 106Cd revisited

Neutrinoless double beta (0ν2β) decays of 106Cd are studied for the transitions to the ground state, 0gs+, and 0+ excited states in 106Pd by using realistic many-body wave functions calculated in the framework of the quasiparticle random-phase approximation and its extensions. Effective, G-matrix-based nuclear forces are used in large single-particle model spaces. Both the β+β+ and β+EC channels of the 0ν2β decay are discussed and half-lives are computed. Particular attention is devoted to the study of the detectability of the resonant neutrinoless double electron capture (R0νECEC) process in 106Cd. The calculations of the present article constitute the thus far most complete and up-to-date investigation of the 0ν2β properties of 106Cd.